500 million year field trip

Dr Jacqueline Owen accompanied new undergraduates on their first field trip where they discovered a Britain with the climate of Barbados.

Last month I travelled with 196 first year students from the new LEC.103 Environmental Processes and Systems module to Kingsdale in North Yorkshire to look at evidence of past climates.

Dr Jennie Gilbert, who led the trip, used the rocks and landscape to reconstruct the climate and environmental conditions for about six different periods through the past 500 million years.

Locality 1 - limestone valley

At our first stop there’s lots of exposure of limestone, the main rock type of the area. Limestone is made up of the fossil remains of tiny sea creatures and coral; the sorts of organisms that live in warm shallow seas.

This tells us that when this rock formed (about 300 million years ago), the UK had a tropical climate similar to Barbados or the Seychelles today - the UK was much closer to the equator then. Unfortunately we’ve drifted a long way north since!

Our stop offers us a good view of the valley. We can see it’s ‘u’ shaped which tells us that this region was probably glaciated in the past. There are also large mounds of sediment along the far side and at the end of the valley (lateral and terminal moraines respectively) which are deposits that have been dumped by the glacier as it retreated.

Locality 2 - a gorge

At our second stop - locality 2 - we can see a gorge. It has steep jagged sides which are overhanging in places, suggesting that its formation was a very rapid process. Further evidence for this comes from large boulders downstream. To rapidly create such a large gorge implies that a lot of water once flowed through here; where did this water come from?

The most likely option is meltwater from the glacier that once filled this valley. But ice melting through climate change is a slow process and this gorge looks like it formed quickly.

One explanation is that the terminal moraine, dumped by the glacier as it retreated, blocked the valley, forming a dam. This would allow a lake to form as the glacier melted, until such a point that the dam failed. If this is the case the gorge may only be as old as the end of the last glacial period (~10,000 years).

Locality 3 - inside a terminal moraine

At locality 3 a landslide has removed the grassy exterior of a terminal moraine, allowing us to see what the deposit really looks like. There’s a large range of rock types, sizes (from boulders to clay) and morphologies but absolutely no sorting. This is because glaciers are anything but fussy. They pick up whatever is in their path, then dump everything unceremoniously as they retreat.

The other telltale sign that this is a glacial deposit is the presence of ‘striae’ which are scratch marks left on the rocks as they have been dragged along.

Locality 4 - a waterfall

The grand finale of our Kingsdale field trip is the beautiful waterfall of Thornton Force. The top half of the waterfall is limestone (indicating warm tropical seas) but the bottom half is a special type of sandstone, called a turbidite, which forms in deep oceans.

When this rock formed, about 500 million years ago, Scotland and southern England were on two separate tectonic plates and Yorkshire was in the ocean that separated them.

There are lots of layers (beds) within the turbidite sequence. These would have been laid down horizontal but are now vertical. This deformation probably occurred when the two tectonic plates converged to create the UK as we know it today. It is likely that mountains were also produced during this process.

However, the contact between the sandstone (turbidites) and limestone is horizontal, suggesting that there was significant erosion between the folding of the turbidites and the limestone forming. A thin layer of conglomerate lies between the sandstone and the limestone and may indicate that powerful rivers were the cause of this erosion.

Past climates

So there we have it: if you were in Kingsdale in the past you may have been in a deep ocean, climbing a mountain, boating down a powerful river, snorkelling in a warm tropical sea, under a glacier or swimming in a flash flood. What will the future hold?

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